1. Field of the Invention
The present invention relates to a method of and device for forming a mask pattern used to form a master mask used in a projection/exposure step carried out to form a metal master serving as a master disk of an optical disk, and a method of producing an optical disk using the metal master formed in accordance with the mask pattern obtained using the method of and device for forming a mask pattern.
2. Description of the Related Art
A general description of the method of producing a master disk of an optical disk is given. The master disk of the optical disk is a metal master or a stamper, a die needed to form the optical disk, and a circular metallic plate which is approximately 0.3 mm thick and has a slightly convex surface for forming, for example, pits therein.
Hereunder, a common method of producing a master disk of an optical disk is described with reference to FIG. 8.
A glass plate is cleaned with a cleaning device, and polished by carrying out a glass plate cleaning/polishing operation 91. Then, a coupling agent which increases the intimacy with which a resist comes into contact with the glass plate is applied by carrying out a coupling agent applying operation 92, after which a photoresist is applied to the glass plate to a certain film thickness using a resist coater by carrying out a photoresist applying operation 93, whereby a resist disk 110 is formed. The resist disk 110 is baked at a certain temperature by carrying out a baking operation 94.
The resist disk 110 that has been baked is set on a precision rotary table 111 of a cutting machine shown in FIG. 9 in order to perform a laser cutting operation 95 thereon. The precision rotary table 111 is rotated by a motor 112 subjected to a rotary servo operation. Then, the resist disk 110 is irradiated with and exposed to a laser beam from thereabove by a linear head 113 constructed so as to be precisely linearly driven on a normal line of the precision rotary table 111, whereby a signal is recorded on the resist disk 110. The linear head 113 comprises a light modulator 114, an optical system lens 115, a beam splitter 116, and an objective lens 117. In the linear head 113, an optical slider 118 is driven as a result of being slid by a radial advancing system motor 119. Authoring data produced by an authoring operation 96 and a signal produced by encoding a subcoding signal by a formatting operation 97, both of which operations 96 and 97 are shown in FIG. 8, are supplied to the light modulator 114 through a signal generator 120. Through the optical system lens 115, the beam splitter 116, and the objective lens 117, the resist disk 110 on the precision rotary table 111 is irradiated with a laser beam emitted from a recording laser 122 and modulated by the light modulator 114 in accordance with the encoded signal. The focusing operation of the objective lens 117 is controlled by a driving coil in accordance with a focus error signal detected by a light detector 121. The radial advancing system motor 119 is subjected to a linear advancing servo operation, while the recording laser 122 is subjected to an exposure power control servo operation.
When the laser cutting operation 95 is completed, a developing device is used to perform a developing operation 98 on the resist disk 110 in order to remove any resist in the pits and grooves, whereby the master disk of the optical disk is produced. When the master disk is produced, a nickel (Ni) electrode with a size not more than 100 nm is formed by an operation 99 carried out to make the master disk electrically conductive. After performing the operation 99, Ni is applied to the Ni electrode to a thickness of approximately 300 (m by a thin-film processor by carrying out an electrotyping operation 100. After performing the electrotyping operation 100, the Ni metallic plate is peeled off from the glass plate, and any photoresist that is stuck is removed in order to adjust the inside and outside diameters thereof, whereby a stamper called a metal master 101 is produced. The precision of the signal of the metal master 101 and the like are evaluated by a signal evaluating operation 102.
However, in the above-described operations, particularly in the laser cutting operation, it is necessary to carefully determine the conditions which allow disks produced after a plurality of later operations of the method have been carried out to satisfy standards. In addition, it is necessary to carefully observe at all times that these conditions are maintained during the long time required to produce the disks.
Pits in different types of media have different pit shapes and sizes, so that the operation conditions need to be separately determined according to the types of media. For example, pits in a compact disk (CD) and a digital video disk (DVD) have forms and sizes given in Table 1.
TABLE 1CDDVDPit length0.9 to 3.3 μm0.4 to 1.87 μmPit width0.5 to 0.8 μm0.4 μm or lessTrack pitch1.6 μm0.74 μmCapacity780 MB4.7 GB
In general, the pit length of a CD is from 0.9 to 3.3 μm, whereas the pit length of a DVD is from 0.4 to 1.87 μm. The pit width of a CD is from 0.5 to 0.8 μm, whereas the pit width of a DVD is equal to or less than 0.4 μm. The track pitch of a CD is 1.6 μm, whereas the track pitch of a DVD is 0.74 μm. The capacity of a CD is 780 MB, whereas the capacity of a DVD is 4.7 GB.
The laser cutting operation using the cutting machine illustrated in FIG. 9 chiefly and ultimately determines the precision of the optical disk, so that driving systems (such as the advancing systems and the rotary system) need to be constructed with ultrahigh precision, an optical system which minimizes the diameter of a laser spot needs to be constructed, and a focus servo capability for controlling the size of a laser spot and a servo controlling capability for controlling the power of the laser at a fixed value in accordance with the rotational speed of the disk are required. This makes it necessary to use an expensive and high-precision machine.
When a pit recording disk, such as a compact disk-read only memory (CD-ROM) or a digital video disk-read only memory (DVD-ROM), is used, the conventional laser cutting operation takes a very long time to complete because the pits of the disk are irradiated with and exposed to laser one pit at a time. For example, when a 4.7 GB DVD is used, the laser cutting operation takes at least 100 minutes to complete. Therefore, the laser cutting operation is considered as one of the operations of the method which takes a long time to complete. During the laser cutting operation, it is necessary to carefully observe that the cutting precision is not adversely affected by eliminating, for example, external vibrations to the extent possible.
In the formatting operation, source media data subjected to an authoring operation is read from a driver, and, in synchronism with the cutting machine, encoding of the data to predetermined codes is carried out in real time. Therefore, the formatting operation also needs to be carried out reliably and precisely for a long period of time. In setting the optical system, various types of optical parts need to be adjusted very precisely every time a different type of disk is used. Therefore, when a differently formatted disk is to be cut with the same cutting machine, changes need to be made in the optical system each time a different type of formatting operation is carried out. Precise adjustments of the optical system require excellence in specialized techniques and time.
In the laser cutting operation, the resist disk exposed to laser is immediately developed by a developing device. Due to unstable process factors such as development conditions, slight changes in the focus servo operation, differences in the laser exposure amount caused by the sizes of pits, etc., a theoretical controlling operation of the laser power alone does not necessarily result in the formation of ideal pits as a result of linear speed and advancing conditions. Therefore, a large jitter value may occur, so that, for example, it may be necessary to precisely adjust the exposure power, to control the power of the laser in correspondence with the sizes of the pits, to control the rising position of the pits (control in the time axis direction), etc.